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#8 If a whole semi load of fireworks went off nearly at once, there may have been no hope for that bridge, or the people thereon.

Something similar to gunpowder is used in fireworks. A truck full of fireworks might have been more than the bridge could take. It doesn't have to blow the bridge into billions of pieces - it just has to deliver a shock wave big enough to collapse it. And it looks like that is exactly what happened. Ultimately, they have to look at regulations for trucks carrying hazardous material.

#9 current seismic practice is to tie both deck sections together with restrainer rods and/or cable (Coalinga/Landers lessons), and install shear haunches so the section don't displace off the pier cap (Northridge lessons). It appears this bridge had neither, so... vertical displacement at gravitational speed plus explosive force. WE learn more from failures than practice